Blender dosn't interpret the Colorspace of the imported Image in the Compositor correctly
Blender does not support ICC profiles - this can be a problem if the image has any ICC profile that is not sRGB. 

If you have a calibrated/profiled monitor, since Blender does not support ICC profiles color would also be sent to the monitor as sRGB instead of the monitor's profile so it would differ from software displaying images correctly taking monitors profile into account. 

If your color management settings are set to filmic or have any looks, the image will be changed according to that.